Within the trucking industry freight trucks traveling on highways have been well known vehicles for many years, the larger of such trucks generally having the form of a tractor pulling a semitrailer and being connected thereto by the combination of a horizontal fifth wheel plate carried by the tractor and a king pin carried on the forward portion of the semitrailer. This truck arrangement permits a pivoting action about the fifth wheel to facilitate maneuvering the truck into small loading zones and this allows for a much shorter turning radius for the entire truck.
In more recent years the trucking industry has been experimenting with the addition of a trailer attached to the rear of the semitrailer. A simple, free coupling between the semitrailer and the trailer is not satisfactory because there is no control over the towed trailer and the forward supporting wheels and axle are free to pivot and thereby to cause "jackknifing" and other linkage difficulties that can endanger the entire combination. Rigid connection arrangements between the semitrailer and the trailer are not acceptable because the truck is too long for desired maneuverability. Typical of such structures are U.S. Pat. Nos. 3,004,772; 3,102,738; 3,288,492 and 3,317,219. It was apparent that a second fifth wheel attachment between the semitrailer and the trailer was necessary. It is not feasible to have a fixed extension structure behind a semitrailer because that would interfere with the rear-end loading and unloading of the semitrailer which is important in being able to service loading docks built to load and unload a semitrailer from the rear. Hence, side loading and unloading have not been adopted, which otherwise would permit an extension from the rear to provide a second fifth wheel.
In general, the prior art has attempted to solve this problem by providing a dolly, which is a short turn wheeled section carrying a fifth wheel plate on its upper surface and has means for a towing connection to the semitrailer ahead and is connected to the trailer behind by means of the second fifth wheel. Such an arrangement is shown in U.S. Pat. No. 3,746,369 where the dolly is an independent vehicular section which is attached to the semitrailer when desired, and in U.S. Pat. No. 3,246,912 where the dolly is a part of the load carrying structure of the semitrailer but which can be detached to become the dolly section. In U.S. Pat. No. 3,374,010 there is shown a semitrailer with an extendible chassis to provide a means for coupling a trailer to a semitrailer through a fifth wheel connection. This arrangement, however, will not satisfy highway transportation laws because for normal loading there are not enough wheels to distribute the load properly. A single pair of wheels at the rear of the semitrailer is moved to support the front of the trailer and no wheels are left to support the rear of the semitrailer. Furthermore, in the normal case the semitrailer has two pairs of rear wheels (two axles) and this patent does not provide for that arrangement. Generally, it would be expected that each of the semitrailer and the trailer would have a double axle arrangement at the rear of the vehicle.
It is to be noted that the use of dollies has many disadvantages, for example, injuries to drivers and hostlers is high, doubles cannot be backed up, the dolly must be licensed and registered, unit numbers and maintenance records must be kept, parking space in terminals must be provided, the dolly must be transported to the place of need, hostle time is longer, lights and flaps must be provided to transport a dolly at the rear of a unit, less stability during braking and in slippery conditions, the dolly may be stolen from open yards, etc. The present invention overcomes these disadvantages by fixedly attaching the dolly to the trailer in a manner heretofore not seen nor taught.
Still further attempts to overcome the above difficulties include U.S. Pat. No. 4,955,629 wherein they provide substantially extendible chassis to expose a fifth wheel to carry the second trailer, and in this arrangement the second trailer is actually carried by the frame of the front trailer, thus, the second trailer actually becomes a part of the load that is carried by the front trailer and therefore is considered to be an "B" train. This arrangement is most unacceptable and includes many drawbacks. Such as, this trailer would have to be built to withstand the pressures on the frame during encounters with uneven surfaces. For example, when the unit encounters a speed bump, the front axle will rise up and thus tend to lift the second axle, causing a downward bending action on the frame where the frame is exposed at the rear of the trailer. When the second axle encounters the bump, it would tend to lift the first axle causing the frame to bend upward in the exposed region, as well as try to separate the frame from the trailer in front of the axle. To counter these problems, the frame would need to be very strong and thus much to heavy. This is very important because trucking companies are very conscious of the empty weight of their units. Therefore it would be most unacceptable to increase the trailers weight by 8000 to 10,000 pounds (as taught within the prior art) which would severely cut into the payload the companies strive for. The present invention recognizes, addresses and resolves this problem by providing a shuttle which rides between the frame rails, with the dolly being far lighter in construction than the telescoping frame as taught within the prior art. The present invention would increase the weight of the frame by substantially only 1000 pounds or less, with the extra axle weighing about 2500 pounds. Therefore when compared to the weight of the cited reference, which is substantially twice as much, the present invention is most advantageous.
Still further within the cited reference, due to the increased weight of the frame, they must use a much larger tire than normal. This is most unacceptable as there are several million loading docks in the country, and they are designed for a standard height trailer, thus the cited reference would necessitate the need for special ramps to accommodate fork lift entry. Concern about highway damage is causing a move toward limiting the weight of an axle with a formula using per square inch of tire contact with the road, thereby discouraging the use of th single tire, as the single tire has less per square inch contact then the combined dual tire arrangement. The State of Washington has started making this change due to heavy Canadian trucks damaging the interstates. Therefore, the present invention overcomes these disadvantages by providing dual wheels which are standard size. Still further, the cited reference is much to costly to produce and it could not be easily installed on trailers which currently use dollies, as can the present invention.
Still further attempts to overcome the difficulties include U.S. Pat. No. 4,365,820 which also addreses the above problems with a sliding frame with a dual axle arrangement that slides rearward. Also, this constitutes a "B" train with the front trailer carrying the rear trailer. The frame would have to be very strong to carry the front trailer with the axles slid beyond the rear of the trailer. The dual axles are also limited to 34,000 pounds instead of 20,000 pounds that each single axle is allowed in most states.